BibTasklets: new tasklet

* to harvest from EDP Sciences
  FTP server.

Signed-off-by: Georgios Papoutsakis <georgios.papoutsakis@cern.ch>
Ah, I understand how it works now. I guess by leveraging the file-system you get some sort of state to recover from should there be any errors. In that case though, the `create_collections` needs to be able to find all the files (maybe already converted in previous runs) between `from_date` and `until_date` (or get a list of). Is that what `convert_files` is giving perhaps?